1. Field of Invention
This invention relates to equipment used for fishing, specifically to a gauge used to determine the depth of fishing lines, lures, and bait.
2. Description of Prior Art
It has been understood among fisherman for many years that a crucial element for success when fishing is to fish at the correct depth in the water. With the advent of devices such as electronic fish finders, the location, including the depth, of fish in a body of water can now be pinpointed precisely. The need for a means to determine the depth of the lure or bait is even more acute than before.
As a gas is compressed, its volume becomes smaller. Robert Boyle defined this relationship between pressure and volume over two centuries ago, proving that this is an inverse and proportional relationship. Water pressure increases directly in proportion to the depth of the water. The combination of these two facts has been used in the prior art as the principle for manometric type depth gauges. The simplest of these gauges would consist of a tube sealed at one end. Above the surface of the water, the open end of the tube allows the air within the tube to be at equilibrium with atmospheric pressure. If the tube is placed into water with its open end down, the air is contained in the tube. As the tube is taken deeper into the water, the increasing water pressure will force water into the tube, compressing the air. The volume of air in the tube at any particular depth is proportional to the water pressure and therefore the depth. If this tube is provided with a scale which is proportioned to compare the air or water volume inside the tube to the corresponding depth, the device becomes an effective depth gauge for water. This type of depth gauge is simple and economical, but is not practical for use by a fisherman without improvement, since the user must read the scale while the gauge is in the water and at a particular depth. Bringing the gauge back to the surface of the water, or altering its depth at all, will change the volume of air inside due to the change in water pressure. Some simple means to record the depth reading at the maximum depth achieved by the gauge is needed, such that a fisherman can read that depth after the device is removed from the water.
Other types of depth gauges, such as those used by divers, could conceivably be attached to a fishing line to determine the depth of the bait or lure. Many of these devices have a marker which records the maximum depth achieved, thus the depth could be read when the device is retrieved from the water. These gauges suffer from several disadvantages: they are complicated and expensive, making them impractical for the above purpose due to the risk of loss when fishing; they are susceptible to damage from impact; and they are not streamlined to move smoothly through water.
Another prior art device is calibrated fishing line, where increments of length along the line are marked or colored such that a fisherman may count the increments as line is fed into the water, thus determining the depth. While this provides a reliable measurement when the line is directly below the fisherman, the disadvantage to this device is that the measurement does not correspond to the correct depth when the line is at some angle other than vertical, such as when it is trolled behind a boat or cast from a shore. It also requires that a fisherman be able to count the increments, which may not be possible when the line is fed rapidly, and the fisherman can easily lose track of the count in any case.
Down-riggers, which are assemblies which lower a heavy weight on a line into the water and include a means to attach a fishing line, generally also include some means to measure the length of line fed out. Disadvantages include: they are large and heavy, requiring attachment to a boat for practical use; a fisherman must operate both the down-rigger mechanism and his fishing pole; and the line drags out at an angle away from the boat when trolling, thus making the length of line an inaccurate measure of depth.
One prior art device, described in U.S. Pat. No. 3,466,787 called a Dynamic Depressor for Fishing Line, purports to assist the fisherman in taking his line to specific depths when trolled. However, the depth this device achieves is influenced by the speed at which it is trolled and the weight of the line and tackle to which it is attached. These limitations are noted on the packaging label for the "Pink Lady," a trademark under which it is marketed. It is not intended to provide an actual measurement of the depth achieved.
A simple solution to providing a reliable depth measurement for the fisherman has thus far escaped the prior art, therefore the prior art is in need of improvement.